An RF (Radio Frequency) system such as a portable phone or a wireless LAN needs a phase adjustment for transmitting signals between devices. A passive element that may be an inductor and/or a capacitor is arranged at an input and/or output part of each device for the phase adjustment. These passive element may be formed as an IPD (Integrated Passive Device) in order to improve the integration density.
The IPD is required to be miniaturized and height-reduced in order to miniaturize and height-reduce the systems that use the IPD. There is a proposal that may meet the above requirements. According to this proposal, an IPD is formed on a main surface of a package substrate, and various types of electronic devices are mounted above the IPD.
Japanese Laid-Open Patent Publication Nos. 2006-157738 and 2007-67236 disclose techniques of forming a passive element on a ceramic substrate. Japanese Laid-Open Patent Publication Nos. 2007-31242 and 2007-123468 disclose techniques of forming a coating layer on a ceramic substrate.
The surfaces of the ceramic substrate have a great roughness, which affects the reliability of the passive elements mounted on the main surface of the ceramic substrate. In a case where an MIM (Metal-Insulator-Metal) type capacitor that is a structural element for IPD is formed on the surface of the ceramic substrate, if the adhesiveness between the lower electrode of the capacitor and the ceramic substrate is not good, the lower electrode made of a metal will be removed from the substrate at the time of annealing. When the surface of the substrate is not flat, the upper and lower metal electrodes of the capacitor may contact each other or the breakdown voltage may be reduced due to the thin dielectric film, whereby short-circuiting may take place.
The roughness of the surface of the ceramic substrate may be planarized by coating such as SOG (Spin On Glass). However, SOG does not resolve warpage of the whole ceramic substrate that takes place in baking, and does not improve the adhesiveness to the capacitor greatly. Warpage of the ceramic substrate may be removed by polishing the surface of the substrate. However, this needs an increased number of fabrication steps and increases the cost.